The present invention is concerned with condensation products based on amino-s-triazines with at least two amino groups and formaldehyde with a relatively high content of sulphonic acid groups and a small content of formate, as well as with the use thereof as additives for inorganic binding agents, especially cement.
It is known that the flowability of building materials based oninorganic binding agents, such as cement, gypsum and anhydrite, can be increased by means of dispersing agents. By means of the improved flowability, a lower water/binding agent ratio can be maintained, whereby higher strengths of the hardened building material result. Reaction products based on melamine, formaldehyde and alkali metal sulphites have, in particular, hereby achieved technical importance.
Thus, in DE-AS 23 59 291, there are described condensation products of melamine, formaldehyde and an alkali metal sulphite in a mol ratio of 1:2.8 to 2.3:0.9 to 1.1. According to conventional ideas, such as are described, for example, in EP-A 0 059 353, melamine and alkali metal sulphite hereby react equimolar with neutralisation of one of the three amino groups of the melamine to give sulphonated methylol compounds which are subsequently further condensed by reaction of the two remaining amino groups of the melamine to give linear polycondensates.
For the achievement of suitable molecular weight distributions, such as are necessary for the dispersing action aimed for, the maintenance of a substantially equimolar mol ratio of melamine and alkali metal sulphite is, therefore, of importance such as is required in the above-mentioned DE-AS 23 59 291 by the ratio of melamine to sulphite of 1:0.9 to 1.1.
This range is only slightly widened according to Japanese Laid-Open Publication 57/100 959 in which molar ratios of melamine:formaldehyde:sulphite of 1.0:2.7 to 3.3:0.9 to 1.2 are disclosed. Since the alkali metal sulphites represent by far the cheapest starting material for the production of sulphonic acid groups-containing melamine-formaldehyde resins, products were also later described in which the ideal 1:1 mol ratio of melamine:sulphite was gone beyond.
Thus, for example, in South African Patent 78/2022, condensation products are described based on melamine, formaldehyde and an alkali metal sulphite with the mol ratio of 1:2.5 to 4.0:0.2 to 1.5. According to EP-A 0 336 165, there is also gone up to the limiting mol ratio of melamine:sulphite of 1:1.5 which also describes sulphonic acid groups-containing condensation products based on amino-s-triazines with at least two amino groups and formaldehyde. It is common to all of these publications that, according to their disclosure, the mol ratio of melamine: sulphite of 1:1.5 is not exceeded.
Flow agents for inorganic building material mixtures are today typical commodity products and, because of the cheaper competition by sulphonic acid group-containing naphthalene-formaldehyde condensation products or lignosulphonates, are under strong price pressure. A further improvement of the price/performance relationship of the melamine-formaldehyde-sulphite condensation products could be achieved by increasing the sulphonate group content since sulphite represents the cheapest component in the production. However, the desired increasing is limited because of the above-mentioned limiting molar relationships, especially of melamine to sulphite.
According to EP-A 0 006 135, it was also suggested to replace the relatively expensive melamine by cheaper urea in the mol ratios of melamine:urea of 0.75:0.25 to 0.35:0.7 but these resins display the disadvantage, that, under the strongly alkaline conditions in cement-containing masses, a cleavage of the condensation products takes place as far as the emission of ammonia from urea. Therefore, for environmental reasons and for the protection of the user, such products are today no longer acceptable.
On the other hand, a further increase of the number of sulphonic acid groups in the condensation product would be regarded as being toxicologically safe but hitherto it has not been possible to realise this. Furthermore, the sulphonic acid group-containing melamine-formaldehyde condensation products according to the prior art have a relatively high content of formate. According to the German guidelines for the granting of permission for concrete additives (wording of June, 1993, Section 6, point 2), certain chemical components of concrete additives are limited. In particular, concrete additives may not contain any materials in such amounts that the concrete or the corrosion protection of the reinforcement (concrete or prestressing steel) could be impaired. In this connection, chlorides, thiocyanates, nitrites, nitrates and formates are set out.
Therefore, it is an object of the present invention to make available condensation products based on amino-s-triazines with at least two amino groups and form-aldehyde with a relatively high content of sulphonic acid groups and a small content of formate which can be produced with simple technical means and are justifiable not only from the economic but also from the ecological point of view.
According to the present invention, this object is achieved by making available condensation products based on amino-s-triazines with at least two amino groups, formaldehyde and sulphite which are characterised in that the mol ratio of amino-s-triazine:formaldehyde:sulphite is 1:2.5 to 6.0:1.51 to 2.0. The content of formate is  less than 0.3% by weight. According to the present invention, these condensation products can be produced by
a) heating amino-s-triazine, formaldehyde and a sulphite in a mol ratio of 1:2.5 to 6.0:1.51 to 2.0 in aqueous solution at a temperature of from 60 to 90xc2x0 C. and at a pH value of from 9.0 to 13.0 until the sulphite is no longer detectable,
b) thereafter, at a pH value of from 3.0 to 6.5 and at a temperature of 60 to 80xc2x0 C., the condensation is continued until the condensation product has a viscosity of 5 to 50 mm2/s at 80xc2x0 C. and
c) subsequently, the condensation product is adjusted to a pH value of 7.5 to 12.0 or a thermal after-treatment is carried out at a pH value of  greater than 10 and at a temperature of 60 to 100xc2x0 C.
The mol ratio of amino-s-triazine:formaldehyde is preferably 1:2.5 to 4.0. The preferred range of amino-s-triazine:sulphite is 1:1.51 to 1.80. Furthermore, a mol ratio of formaldehyde:sulphite of 1.8 to 2.4:1 is preferred.
Surprisingly, has, namely, been shown that, in this way, condensation products can be produced with outstanding liquefying action and high solids content which have an excellent storage stability.
The fact that the condensation products according to the present invention have a relatively low content of formate, which arises during the alkaline condensation reaction from formaldehyde by a Cannizzaro reaction, must also be regarded as being a further surprising effect.
In the first reaction step a), amino-s-triazine, formaldehyde and a sulphite are pre-condensed in aqueous solution at a temperature of from 60 to 90xc2x0 C. and at a pH value of 9.0 to 13.0 until the sulphite is no longer detectable. The adjustment of the pH value is carried out with the usual alkaline-reacting compounds and preferably with an aqueous solution of sodium hydroxide.
Melamine is preferably used as amino-s-triazine but guanamines, for example benzo- or acetoguanamine, can also be used.
In the scope of the present invention, it is also possible to use a mixture of amino-s-triazine with other aminoplast formers, in which case up to 50 mol % of the mixture can consist of other amino-plast formers, for example urea, thiourea, dicyan-diamide or guanidine (salts). In the case that a part of the amino-s-triazine has been replaced by other aminoplast formers, this constitutes a part of the mol ratio.
Formaldehyde is preferably used in the form of a 30% aqueous solution (formalin). However, any other form can also be used, for example paraformaldehyde.
As sulphite derivatives, there are preferably used bisulphites or pyrosulphites. However, the corresponding alkali metal and alkaline earth metal sulphites can also be used.
The mol ratio of amino-s-triazine, formaldehyde and alkali metal sulphite in an amount of 1:3.0 to 6.0:1.51 to 2.0 is important for the present invention. Only in this way is it possible to produce condensation products with a relatively high content of sulphonic acid groups and a low content of formate. The mol ratio of amino-s-triazine to formaldehyde to sulphite is preferably 1:3.0 to 4.0:1.51 to 1.80 and the ratio of formaldehyde:sulphite is from 1.8 to 2.4:1. Since, in the scope of the process according to the present invention, the starting materials used are incorporated practically completely into the condensation products obtained, the molar composition thereof corresponds to the mol ratio of the starting materials. According to a preferred embodiment, the concentration of the starting components is so adjusted that the solids content of the end product is 30 to 60% by weight and preferably 40 to 50% by weight.
Subsequent to the alkaline pre-condensation (reaction step a)), in reaction step b), further condensation is carried out in an acidic pH range, namely, at a pH value of from 3.0 to 6.5 and at a temperature of 60 to 80xc2x0 C. The adjustment of the pH value hereby takes place with the conventional acid-reacting compounds or salts and especially with mineral acids. For reasons of cost, sulphuric acid is hereby especially preferred.
The acidic condensation reaction is ended when the viscosity of the aqueous solution at 80xc2x0 C. has reached values of from 5 to 50 mm2/s and preferably of 5 to 25 mm2/s. The measurement of the viscosity hereby takes place with conventional viscosity measurement apparatus, for example an Ubbelohde viscosimeter. To stop the acidic polycondensation reaction, according to reaction step c) a pH value of 7.5 to 12.0 is adjusted which, again, can take place with conventional alkaline-reacting compounds and preferably with an aqueous solution of sodium hydroxide.
In the scope of the present invention, it is also possible, instead of the pH value adjustment, to carry out a thermal post-treatment of the condensation products in an alkaline range at a pH value of xe2x89xa710 and at a temperature of 60 to 100xc2x0 C. By means of this post-treatment step, the content of free formaldehyde, which is normally from about 0.2 to 0.3% by weight, can be still further reduced and, depending upon the period of time of the treatment (as a rule, 0.5 to 3 hours), can be adjusted to  less than 0.2% by weight and especially of  less than 0.1 by weight, whereby, however, the formate content can increase slightly.
The condensation products produced in this way have a relatively low content of formate of  less than 0.3% by weight and even in the case of a high solids concentration are storage-stable, i.e. they can be stored for at least a year at ambient temperature without problems. Because of their good liquefying action, the condensation products according to the present invention are outstandingly useful as additives for inorganic binding agent suspensions and especially for those based on cement, lime and gypsum, in which case they can be used in an amount of 0.1 to 10% by weight and preferably of 0.1 to 5% by weight, referred to the binding agent content of the corresponding hydraulitically hardened building material mixtures (for example mortar, concrete or gypsum).